To change the cover photo for your Sonlet Party, go to your Parties page, and click the Menu button to the left of the Party name. Select Edit Details.
This will take you to a page you can edit the details of the party, including the cover photo.
If you have changed the cover photo for your Sonlet Party, but when you post the link to Facebook, it still shows the old photo on the link preview, here are some troubleshooting ideas:
This happens due to Facebook caching your old photo in association with your photo link. You can reset this by pasting your Party link in the "debug" box at this link: https://developers.facebook.com/tools/debug/
Once you paste your Party link into the box, press debug. Then, look down at the table of information below for the row that says "time scraped" and hit the button that states "scrape again". This should get Facebook to release the previously stored picture and show the new cover photo for future posts of your Party link.